It is proposed to investigate the structures of several important naturally-occurring substances via single-crystal X-ray diffraction techniques. The substances to be studied fall under two main categories: a) the sideramines and other hydroxamate complexes, and b) uric acid and urate salts. The sideramines are an important class of crystalline, iron-containing growth factors of bacterial origin. They are believed to act as iron-transporting agents and suppliers of iron for heme synthesis. In their deferri form (i.e., without the iron atom), these compounds have important therapeutic applications toward the control of iron poisoning and various metabolic disorders involving iron. A related compound, mycobactin, is a substance crucial to the growth of mycobacterium tuberculosis. By determining the molecular geometries of these important compounds, we hope to uncover structure-activity relationships that might be useful in understanding the processes of iron transport. We also hope that a knowledge of the structures of these growth factors will aid the development of future antibiotics for microbes such as m. tuberculosis. Uric acid and the urate salts are colorless crystalline degradation products of the purines. Because of their low solubility in aqueous media, they accumulate as unwanted solid deposits in certain metabolic disorders such as gout, and are also among the major constituents in kidney and bladder stones. It is hoped that a knowledge of their molecular structure, the molecular packing in the crystal and their crystal growth behavior will help in understanding the factors behind the initiation and growth of these important compounds.